1. Field of the Invention
This invention relates to a method for preparing aqueous anionic polyurethane dispersion adhesive compositions which provide good heat and water resistance.
2. Description of the Prior Art
Aqueous dispersions of polyurethanes are well known. Dispersion may be accomplished by use of an external surface active agent or by providing the polyurethane polymer backbone with non-ionic ethylene oxide ether moieties or by providing the polyurethane polymer with neutralized cationic or anionic groups.
For anionic aqueous polyurethane dispersions, the ionic salt groups are salts of carboxylic acid or sulfonic acid groups. In the preparation of anionic polyurethane dispersions it is generally preferred to prepare a polyurethane prepolymer having a small residual free isocyanate content, disperse the prepolymer in aqueous media, and then add a plural functional relatively low molecular weight primary and/or secondary amine as a chain extender. This chain extension process is needed because a higher molecular weight poly(urethane/urea) having better heat resistance is obtained after extension. To aid in dispersion of the isocyanate functional polyurethane prepolymer and sometimes to control the polyurethane forming reaction, a volatile solvent such as acetone, methyl ethyl ketone, N-methyl pyrrollidone or the like are employed in the prepolymer forming reaction. If the solvent level is high after dispersion, it usually must be substantially removed by distillation, a complicated process step requiring expensive equipment. Furthermore, the solvent removal step results in poor volume/time yield in the manufacturing process and the handling of large volumes of solvent in the reaction medium and in the removal step increases the risk of explosion or fire during the manufacturing process.
Polyurethane prepolymers produced in the manner just described are reaction products of a plural isocyanate compound (polyisocyanate) and a polyol component which includes a polymeric polyol such as a polyether polyol or a polyester polyol and, typically, a monomeric or very short chain oligomeric polyol. The polyurethane so produced will have a --(--A--B--)-- type block polymer structure where the A segment is relatively soft segment derived from the polymeric polyol and the B segment is a hard segment derived from the polyisocyanate component and any monomeric or short oligomeric polyol in the polyol component. For nearly all of the commercial anionic aqueous polyurethane dispersion products, the ionic groups are contained in the hard segment.
In considering how to manufacture polyurethane dispersion products it must also be kept in mind the desired use properties of the final polymer. In the case of dispersions used as adhesive products, heat and water resistance are important properties which are significantly affected by the ingredients of the polyol component employed. For instance, increasing the total amount of ionic groups in the hard segment of the polymer can result in reduction of the relative amount of soft segment and reduce water resistance and flexibility and elasticity of the final polymer. Adding components which include non-ionic ethylene oxide ether or other polyether moieties may improve dispersability but likewise at the sacrifice of heat and water resistance.
Henning, et al., U.S. Pat. No. 4,870,129, discloses use of the sodium salt of N-(2-aminoethyl)-2-aminoethane sulfonic acid (AAS salt) to prepare polyurethane dispersions. The aqueous polyurethane dispersions are reported to have exhibited good stability at low pH values (5-7), but only medium heat resistance. Commercial products based on this monomer, such as Bayer KA-8464 show poor water resistance. Furthermore the process employed in this reference employs a level of acetone solvent, before removal by distillation, which is 80 weight percent of the dispersion. Removal of this amount of solvent is a very complicated process on a commercial scale. The sulphonate groups are incorporated into the hard segment when this compound is employed.
Reiff, et al, U.S. Pat. No. 4,108,814, discloses aqueous sulfonated polyurethane dispersions based on sulfonated polyether diols and a water soluble sulfonated diamine in a solvent free process. However, this solvent-free process produces a polymer which will not have a high crystallization rate, high heat resistance or good water resistance.
U.S. Pat. No. 5,334,690, to Schafheutle, et al, discloses a preparation of a polyurethane dispersion which employs a polyester or polyether diol having anionic groups. The reference indicates that the polyurethane may be prepared in the presence of a solvent or in a solvent-free melt process. In examples this reference utilizes a carboxylated polyester polyol and acetone at a level of about 40% is employed. A melt process is also exemplified using a sulfonated polyester polyol. However the high temperature used in a melt process can introduce side reactions such as diisocyanate reaction with carboxylic acid and/or urethane groups and even trimerization of the isocyanate so that the final product may not have a good linear structure and therefore the polymer may have poor crystallization rate, poor green strength, and inadequate heat resistance. Therefore a melt process is a less preferred process for preparing aqueous polyurethane dispersions.
A solvent-free or low solvent process for preparation of aqueous polyurethane dispersions is disclosed in U.S. Pat. No. 4,829,122, to Pedain, et al, but this process utilizes a blocked amine chain extender.
Duan et al in copending application Ser. No. 08/343,676, filed Nov. 22, 1994 as a continuation-in-part of application Ser. No. 08/126,508, filed Sep. 24, 1993, now abandoned, disclose aqueous polyurethane dispersions based on use of both sulfonated polyester polyols and a hydroxy carboxylic acid, suitably an .alpha.,.alpha.-dimethylol alkanoic acid such as dimethylolpropionic acid, which gives a polyurethane polymer having anionic groups on both hard and soft segments of the polymer. Acetone and solvent-free processes are used to prepare the polyurethane prepolymers and, in the case of the acetone process, after dispersion and chain extension the acetone is distilled off. In the solvent-free process the polyesters used in these formulations include ethylene oxide ether moieties due to use of diethylene glycol in the polyester and have a melting point below 50.degree. C. and preferably ones which are liquid at room temperature. The polyurethanes produced in accordance with the solvent-free process have poor heat and water resistance.
Duan et al in copending application Ser. No. 08/304,653, filed Sep. 9, 1994, disclose aqueous polyurethane dispersions based on use of both sulfonated polyester polyols using a water compatible solvent such as acetone, NMP or DPMA and sulfonated polyesters of higher molecular weight and a low molecular weight non-carboxylated diol which has high heat resistance and good water resistance. In the case where NMP or DPMA are used, a range of about 3-15% solvent based on final dispersion weight is disclosed. In the case where acetone is used, after dispersion and chain extension, the acetone is distilled off so that the final level of acetone may be as little as 1.0% acetone or even less.
Surprisingly it has now been discovered that similar polymers to those of copending application Ser. No. 08/304,653, having high heat and water resistance, can be produced by a low acetone solvent process which allows for good control of the prepolymer forming reaction but which does not incorporate so much solvent that it must be removed from the final dispersion.